Luminescent materials having long decay periods ranging from a few minutes to several hours are known and typically produce their radiation by phosphorescence. Such phosphorescent materials have typically been used in safety signs or on watch or clock dials. In recent years, technology has developed which make it possible to imbed luminescent materials in pressed or molded plastic products. Such technology significantly broadens the range of long decay phosphor applications.
Copper-activated zinc sulfide, such as ZnS:Cu,Cl, is frequently used for these long decay phosphor applications because copper-activated zinc sulfide produces emission in a spectral region having a relatively high luminous efficiency. However, the properties of copper-activated zinc sulfide are not completely satisfactory since the brightness of the phosphor falls off substantially after extended decay periods, such that the emission is barely perceptible after about 30 minutes. Furthermore, copper-activated zinc sulfide is subject to degradation and deterioration when exposed to UV radiation in a moist or humid atmosphere. The body color of the material containing the zinc sulfide darkens, possibly due to the presence of elemental zinc on the surface. Use of such materials for outdoor applications has, therefore, been severely limited.
Phosphorescent materials having a long decay period may be used in such fields as the graphic arts, interior decorating or printing inks. For these applications, alkaline earth sulfide phosphors have been used, since they can be prepared with a broad gamut of colors ranging throughout the visible spectrum from blue to red. These materials, however, are hygroscopic and react readily with moisture tending to generate hydrogen sulfide, a noxious and toxic substance. These properties severely restrict their use in the home.
Recently, long decay phosphors comprised of rare-earth activated, divalent, boron-substituted aluminates have been developed, U.S. Pat. No. 5,376,303. These long decay phosphors overcome some of the disadvantages of ZnS:Cu,Cl and give long and bright emission in the range from blue to yellow-green. This family of phosphors is activated with divalent europium as a luminescent center. However, divalent europium has not been used to produce orange or red emission in this family of host crystals.
In the past, (Zn,Cd)S:Cu,Cl or CaS:Eu have been used for orange or red emitting long decay phosphors. However, the properties of these materials have not been satisfactory because of the above mentioned disadvantages of sulfide compounds. In particular, these sulfide materials are subject to degradation and deterioration when exposed to UV radiation in the same manner as ZnS:Cu,Cl. In addition, these materials contain the toxic element cadmium. CaS:Eu provides red emission, but this material readily reacts with moisture and tends to generate H.sub.2 S gas into the atmosphere. Thus, due to the characteristics of these sulfide materials, they are troublesome and difficult to process and undesirable to use in finished products. Therefore, chemically stable and non-toxic red emitting long decay phosphors are still desired.
Divalent titanates such as strontium titanates or barium titanates are well known as dielectric substances and thermistor materials. The possibility of using divalent titanates as the host crystal for a luminescent material has also been studied. The emission of various activators in divalent titanates are reported in several studies; "Optical spectra of rare earth activated BaTiO.sub.3 ", The Journal of Chemical Physics, Vol. 31, No.5, 1272-1277, November 1959, Seymour P. Keller and George D. Pettit; "Luminescence of Sm.sup.3+ in BaTiO.sub.3 Matrix", J. Phys. Chem., Solids Pergamon Press, Vol. 23, 749-757, 1962, S. Makishima, K. Hasegawa and S. Shionoya; and "Photoluminescence of Cr doped CaTiO.sub.3 ", Physical Review B, Vol.2, No.11, 4351-4353, December 1970, L. Grabener and S. E. Stokowski. None of these studies disclose red emitting long decay phosphors using a divalent titanate as the host material.